The present invention relates to the field of magneto-optic recording. More particularly, it relates to improvements in magneto-optic recording elements of the type having a direct-overwrite capability.
Conventional magneto-optic disks require two revolutions of the disk to record information. The first revolution is used to erase any previously recorded information, while the second revolution is used to record the new information. The information is stored as patterns of vertically oriented magnetic domains arranged along an annular track on the magneto-optic disk. Traditional magnetic recording systems have a direct-overwrite capability in that erasure of previously recorded information is intrinsic in the recording process. Hence, magnetic systems require only one revolution to record data. As a consequence, compared to magnetic disks, magneto-optic disks are disadvantageous from the standpoint of requiring a substantially longer effective access time.
In U.S. Pat. No. 4,855,975 to Saito et al, a magneto-optic recording system is described which eliminates the noted two-revolution requirement. Here, the recording element comprises two different magnetic layers or films laminated together. One layer, the reference layer, has a low room-temperature coercivity H.sub.c(ref.) and a high Curie temperature T.sub.c(ref.). The other layer, the so-called memory layer, has a high room-temperature coercivity H.sub.c(mem.) and a low Curie temperature T.sub.c(mem.). As the disk-shaped recording element rotates, the disk passes in close proximity to an initialization magnet, thereby exposing the disk to a field -H.sub.i perpendicular to the disk surface, where H.sub.c(mem.) &gt;H.sub.i &gt;H.sub.c(ref.). This initialization field serves to vertically orient all magnetic domains of the reference layer in a given direction (e.g. "down") but has no effect on the memory layer. A second magnet, the bias magnet, is arranged to expose the area of the disk which is selectively heated by an intensity-modulated laser to a field H.sub.b, where H.sub.c(mem.) &gt;H.sub.c(ref.) &gt;H.sub.b. The bias field H.sub.b is perpendicular to the disk and directed in the direction opposite to that of -H.sub.i. While the disk is being read, H.sub.b has no effect on either layer.
When the above-described disk is exposed to a certain power of laser light, the memory layer will be heated above its Curie temperature, while the reference layer remains below its Curie temperature. Under these conditions, the magnetic exchange interaction which exists between the two layers will cause the magnetization of the memory layer to be aligned with the magnetization of the reference layer. Whenever the disk is selectively exposed to a higher light power, both layers become heated to temperatures above their respective Curie temperatures, and the magnetization of the heated portions of both layers become realigned in the direction of the bias field H.sub.b, i.e., the field produced by the bias magnet. Consequently, by modulating the laser light intensity between these two power levels, digital information can be recorded while simultaneously erasing any pre-existing information.
Successful direct overwriting using the above-described recording element requires a substantial Curie temperature differential (e.g. at least 50.degree.) between the two magnetic layers, as well as a substantial difference in magnetic coercivity (e.g. at least 5000 Oersteds). Such temperature and coercivity differentials have been achieved in the prior art recording elements by either using entirely different magnetic materials as the memory and reference layers (e.g. by using a GdDy alloy as the memory layer and a GdTb alloy as the reference layer) and by adjusting the stoichiometry of the same elements used in both layers (e.g., by varying the concentrations of terbium and iron in a TbFeCo layer). While the requisite coercivity differential can be readily achieved by these techniques it has been difficult to achieve a Curie temperature differential substantially greater than about 70.degree. C.